A method for attaching a user terminal to a base station of a network, also called user association, well known by the man skilled in the art, comprises the step of attaching the user terminal to the closest base station.
One problem of the well-known prior art is that this may lead to unbalanced load, especially among heterogeneous types of base stations (some with low maximum transmit power, and some with high maximum transmit power), when small cells and macro cells co-exist. Another problem is that high-speed user terminals attached to small cells need to handoff frequently which results in extra cost of resources used for handover (for example such as operation overhead during which no data transmission is performed but only handover).
Moreover, it results also in low spectrum utilization efficiency. Indeed, when a user terminal handoffs, there is a time gap due to switching from one base station to the other. In this time gap (duration), data transmission has to be held. For example, if this time gap has to be 1 second, however, the switching/handoff will happen regularly immediately after every 1 second of data transmission, then the time efficiency is only 50% (given by 1 s/2 s).